After having installed a fresh WP in one of your website folders, go through the following list:

• WP settings (paths, timezone, SSL settings, etc.)
• PHP version check: if the PHP version is outdated, check the PHP settings of your domain (via your hoster) and change the version there to the latest one
• database settings: phpAdmin, wp-config
• user settings: name, email, role/rights, password
• theme installation: install a theme of your choice (one backup theme required)
• plugins: only most important and most necessary (keep them updated)
• theme customizer: only absolute basics here, everything else in the child theme